1. Field of the Invention
The present invention is generally in the field of fabrication of semiconductor devices. More specifically, the invention is in the field of fabrication of bipolar transistors.
2. Related Art
Gallium arsenide (“GaAs”) based devices are able to provide the power and amplification requirements of various applications, such as handset power amplifiers, with improved linearity and power efficiency. Of particular interest are GaAs heterojunction bipolar transistors (“HBT”), which exhibit high power density capability, making them suitable as low cost and high power amplifiers in handsets used in CDMA, TDMA and GSM wireless communications. However, handset power amplifiers are required to operate over a large temperature range of between approximately −30.0° C. and approximately 85.0° C. Additionally, handset power amplifiers are generally powered by a battery, which can vary in output voltage. As a result, handset power amplifiers are required to effectively operate with a supply voltage that may range between approximately 2.65 volts and approximately 5.0 volts. Thus, semiconductor manufacturers are challenged to provide GaAs HBTs that must operate effectively in the above temperature and voltage ranges in devices such as handset power amplifiers.
By way of background, as temperature decreases, the base-emitter turn-on or threshold voltage and band gap of a GaAs HBT increases. In a handset power amplifier comprising a GaAs HBT, the turn-on voltage of the GaAs HBT can be utilized to determine a reference current, which can be utilized to set the current in the handset power amplifier. As a result, as temperature decreases, the current flowing though the handset power amplifier decreases, which undesirably reduces the performance of the handset power amplifier.
In an attempt to reduce the turn-on or threshold voltage of the GaAs HBT to enable the GaAs HBT to operate more effectively under wider temperature and supply voltage ranges, some semiconductor manufacturers have added a small amount of nitrogen to the base of the GaAs HBT. As a result of adding nitrogen to the base, the band gap of the GaAs HBT is reduced, which reduces the turn-on voltage of the GaAs HBT. For example, by adding approximately 3.0 atomic percent of nitrogen to the base of the GaAs HBT, the band gap of the GaAs HBT can be reduced by approximately 100.0 millivolts. However, the nitrogen also undesirably reduces DC current gain of the GaAs HBT.
In order to counter the reduction in DC current gain caused by the nitrogen, some semiconductor manufacturers have attempted to add indium to appropriately alter the band structure of the base to increase DC current gain. However, when the base is carbon doped, which is desirable for HBTs, the addition of indium in the base causes carbon to be compensated out of the base. Consequently, the addition of indium undesirably reduces DC current gain and linearity of the carbon-doped GaAs HBT. As a result, the addition of indium undesirably degrades performance the carbon-doped GaAs HBT.
Thus, there is a need in the art for a GaAs HBT having increased gain, linearity, and performance.